The present invention relates to electrophotographic light-sensitive elements.
Previously known electrophotographic light-sensitive elements employed amorphous selenium; photoconductive materials obtained by doping impurities such as arsenic, tellurium, or antimony in, for example, amorphous selenium; or a dispersion of photoconductive materials such as zinc oxide or cadmium sulfide in resin binders. These light-sensitive elements have disadvantageous properties, however, in that they exhibit poor stability to environmental conditions such as temperature and moisture, mechanical stress, or environmental contamination.
In recent years, various techniques for eliminating the problems of these conventional electrophotographic light-sensitive elements have been proposed which use amorphous silicon (a-Si) as the photoconductive material. The a-Si prepared by deposition or sputtering is not desirable as a photoconductive material for electrophotographic light-sensitive elements, since it has a relatively low dark resistivity of 10.sup.5 .OMEGA.-cm and a very low photoconductivity. This is due to the formation of so-called dangling bonds upon cleavage of Si--Si bonds in a-Si prepared by such a process. Because of such defects, many local levels are present within the energy gap, and therefore conduction via thermally excited carriers can occur even when the material is in the dark. Consequently, the dark resistivity is small. Further, upon exposure of the material to light, photoexcited carriers are trapped in the local levels so that photoconductivity is poor.
In amorphous hydrogenated silicon (a-Si:H) prepared by glow discharge decomposition of silane gas (SiH.sub.4), the above-described defect is alleviated by the presence of hydrogen atoms bonded to the silicon, and the number of dangling bonds can be greatly reduced thereby. The photoconductivity of such materials is greatly improved, and electron valency of P type and N type can also be controlled. The dark resistivity of these materials, however, is from 10.sup.5 to 10.sup.9 .OMEGA.-cm at the highest, which is still low when compared to resistivity values of greater than 10.sup.12 .OMEGA.-cm required for effective electrophotographic light-sensitive elements. Accordingly, light-sensitive elements comprising a-Si:H have a large dark decay rate of the surface potential and a low initial charge potential.
To impart a charge retention capability to such a-Si:H, the dark resistivity is enhanced to 10.sup.12 .OMEGA.-cm or more by doping the a-Si:H with an appropriate amount of boron, paying attention to the above-described fact that electron valency can be controlled. This technique enables utilization of a-Si:H light-sensitive elements in a copying process of the Carlson type.
The light-sensitive elements using boron doped a-Si:H as the surface layer provide good copying images at an initial stage, but often result in poor images when copying is made after they are stored in the air or under high humidity over long periods of time. Further, it is found that repeated copying gradually causes blurring of the images. It is established that such deteriorated light-sensitive elements are liable to produce blurred images at high humidity and, further, that as the number of copies made increases, the critical humidity at which blurring of images occurs tends to become lower.
As described above, the light-sensitive elements having a-Si:H as the surface layer are prone to be affected at the uppermost surface by exposure to the atmosphere or moisture over long periods of time, or by exposure to chemical species (ozone, nitrogen oxides, nascent oxygen, etc.) produced by corona discharge during the copying process. It is thus considered that deterioration in image quality might be caused by a chemical change, but the mechanism of deterioration has not been sufficiently studied so far. In order to prevent the deterioration of image quality and improved printability, a method for achieving chemical stability by providing a protective layer at the surface of the light-sensitive element has been attempted. For example, there is known a method for preventing deterioration of the surface layer of the light-sensitive element due to repetitive copying or environmental conditions which comprises providing a surface protective layer of amorphous hydrogenated carbonated silicon (a-Si.sub.x C.sub.1-x :H, 0&lt;x&lt;1) or amorphous nitrogen hydrogenated silicon (a-Si.sub.x N.sub.1-x :H, 0&lt;x&lt;1). (Published unexamined Japanese Patent Application No. 115559/82.) These surface layers, however, do not provide resistance to moisture under highly humid conditions for extended periods of time, although printability of the light-sensitive element can be improved to a considerable extent by appropriately choosing the carbon concentration or the nitrogen concentration in the surface protective layer. After copying is repeated several ten thousand times, blurred images still occur at 60% relative humidity or higher. Further, when the amount of carbon is increased in an attempt to enhance the protective function, other problems in characteristics, e.g., increase in residual potential, are caused. Therefore, it is the state of the art that the printability and moisture resistance of light-sensitive elements cannot be greatly improved by providing these surface protective layers while the characteristics of the light-sensitive elements are satisfactorily maintained.
It is an object of the present invention to provide electrophotographic light-sensitive elements having excellent durability, printability, and resistance to moisture, which eliminate the foregoing problems. The light-sensitive elements according to the invention do not exhibit deterioration phenomenon during storage nor in repeated use, and there is little reduction in such characteristics as image quality, even in a highly humid atmosphere. Thus, the electrophotographic light-sensitive elements according to this invention provide stable characteristics as light-sensitive elements and have little restriction upon environments for use.